JPH0363703B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for quantitative electroanalytical measurement of elemental hydrogen and a sensor for quantitative electroanalytical measurement of elemental hydrogen. Electrochemical cells of this type for use in quantitative electrochemical analysis are well known and generally include an operating sensing electrode with a defined or electroanalytically effective surface, a counter electrode, and optionally a guard electrode. , an electrolyte in contact with the electrodes, and a membrane substantially impermeable to the electrolyte but permeable to the electroactive substance to be measured; , defines the sensor surface as a barrier between the electrolyte space (in particular the electrolyte film present on top of the sensing electrode) and the surrounding medium containing the electroactive substance to be measured. To carry out amperometric analysis operations, the operating electrodes of this type of cell device are polarized by a constant DC voltage.
A current is generated that is proportional to the activity of the electroactive substance being measured in steady state. This type of cell and its operation and use for measurement purposes are
For example, U.S. Pat.
No. 3223608, No. 3227643, No. 3325378, No.
No. 3372103, No. 3406109, No. 3429796, No.
3515658, 3622488 and 4096047, and UK Patent Application No. 1013895. Considered a pioneer invention, Leland E.
The aforementioned U.S. Patent No. 2913386 granted to Clark
No. 1 already teaches that a method of this kind is suitable for use in measuring electroreducing or electrooxidizing gases with an electroanalytical device, also called a "membrane-coated polarographic detector". There is.
Since the term "polarographic" is also used for techniques based on dropping mercury electrodes and operated in a voltammetric or galvanic manner, the term "membrane-coated amperometric cell" (MEAC) is used herein. For example, the term is used to refer to electroanalytical probes such as ``Clark cells'' and their modified probes, which use protective electrodes and various devices to improve operability, reliability, sensitivity, and maintenance. include. However, although the measurement of electrolytically reducing gases (particularly oxygen) with MEAC sensors is widely accepted [see e.g. Previous attempts to use this technique to measure hydrogen (particularly hydrogen) have not met with reasonable success. To the best of the applicant's knowledge, the measurement of hydrogen using a MEAC type sensor was first carried out by Sawyer et al. [Analytical Chemistry, Vol. 31, p. 2 (1959)].
It is reported here that attempts to detect hydrogen using this principle were completely unsuccessful. M.D. Green et al. then modified the above-mentioned U.S. Pat.
Discloses a method for hydrogen detection by MEAC, and this US patent is applicable to various fields of technology where hydrogen detection is important, such as using hydrogen as a reactant, coolant or fuel, and especially for explosive mixtures of hydrogen and air. It describes technology to detect hydrogen leaks to prevent hydrogen formation. Although the need for hydrogen detection and quantitative measurement has never diminished, electroanalytical methods using MEAC-type sensors have generally proven unsuitable, and hydrogen sensors have not been commercially available to date. Green et al., as well as various other reports [Huon-Gruniger et al., Elvetica Himica Acta, Vol. 61, (1978), p. 2375; Srinivasan et al., Analytical Chemistry, Vol. 53;
(1981), p. 928; Mills et al., Analytical Chemistry, Vol. 53, (1981), p. 1254; Niedratsuha et al., Analytical Chemistry, Vol. 54
Vol., (1982), p. 1651] A common feature of conventional hydrogen sensing by MEAC is that for stability of the measurements and especially the absence of interfering drifts, the anodic sensing surface is generally made of metallic platinum. It is necessary to enlarge the surface to some extent by the action of microcrystalline layers of the kind obtained by etching techniques or by electrolytic platinization or deposition techniques including layers of platinum black. As disclosed in US Pat. No. 3,325,378, platinum black is preferred as an anode sensing surface for etched platinum surfaces. Because,
This provides a more stable output, as well as a surface increase or "roughening" required for the anode electrode of a MEAC-type hydrogen sensor, which results in an enlarged platinum sensing area and activation, compared to traditional polished, i.e. nearly smooth, This is consistent with the teachings of the art which precludes the use of metallic platinum anodes for this purpose. The use of smooth polished electrode surfaces of noble metals, including platinum, is quite conventional in MEAC-type sensors for oxygen measurements, and the use of smooth polished platinum surfaces in the sensing electrodes as platinum blacks or other area enlargers. The advantages of using a platinum surface are obvious to those familiar with electrolysis technology. The reproducibility and sensitivity of the sensing area, as well as the ease of manufacturing, maintenance and maintenance, are significantly better than using a smooth polished platinum surface; These advantages would not be foregone unless one is convinced that the use of coated platinum surfaces cannot be completely avoided. As is clear from this, conventional hydrogen sensing methods using MEAC sensors appeared inoperable due to the smooth anodic sensing surface of metallic platinum if an output with acceptable stability was to be obtained. Therefore, the main objective of the present invention is to
An object of the present invention is to provide an improved method for the quantitative electroanalytical measurement of elemental hydrogen in which the anodic hydrogen sensing electrode has a substantially smooth sensing surface and provides an output with acceptable stability. Furthermore, it is an object of the present invention to provide an improved method that provides simplicity, accuracy, stability, simplicity and convenience in the operation of MEAC type sensors for quantitative electroanalytical measurements of elemental hydrogen. Another object of the invention is a MEAC type sensor for quantitative electroanalytical measurements of elemental hydrogen having a smooth polished anode sensing surface made of metallic platinum and yet providing an output with acceptable stability. The goal is to provide the following. Other objectives will become apparent from the description below. According to the invention, the aforementioned object comprises at least two electrodes which are in contact with an aqueous electrolyte and provide a reactive impedance of the amperometric cell, which are permeable to elemental hydrogen but substantially permeable to the electrolyte. The electrolyte is separated from the fluid medium by a membrane that is impermeable to The above-mentioned at least two electrodes are composed of an anode hydrogen sensing electrode and a cathode counter electrode, the aforementioned hydrogen sensing electrode has a substantially smooth sensing surface made of a platinum group metal, The smooth sensing surface is 1:1~
Elemental hydrogen having an apparent surface area:real surface area ratio in the range of 1:3 and selected such that the aforementioned membrane impedance sufficiently exceeds the reaction impedance to effect membrane control operations of the amperometric cell. Quantitative electroanalytical measurement method, and an amperometric cell comprising an anode hydrogen sensing electrode and a cathode counter electrode in contact with a hydrogen electrolyte and having a reactive impedance, and a current measurement cell permeable to elemental hydrogen but substantially impermeable to the electrolyte. is impermeable to
and a membrane providing a membrane impedance. The aforementioned smooth sensing surface has an apparent surface area:real surface area ratio in the range of 1:1 to 1:3, and the aforementioned membrane has a thickness in the range of 10 to 300 μm and a thickness of 9.8×10 ² (μmol・
This is achieved with a sensor for quantitative electroanalytical measurement of elemental hydrogen with a hydrogen permeability coefficient of less than μm·m ⁻² (Kg/m ² ) ⁻¹ ·s ⁻¹ ). The foregoing objects and other advantages of the present invention are now achieved by: (a) the anodic hydrogen sensing electrode has a substantially smooth sensing surface made of substantially platinum metal; and (b)
It has been found that this can be achieved by maintaining the membrane impedance of the MEAC sufficiently above the reaction impedance to provide membrane-controlled (ie, diffusion-controlled) operation of the MEAC. Although this explanation is abstract and supported by relatively complex mathematical processing as explained below, its practical implications are quite simple and effective. In fact, this new hydrogen sensing method can be implemented using essentially conventional sensors simply by selecting membranes with low hydrogen diffusion according to easily determined criteria as described below. Of particular interest is the problem that, for stable hydrogen sensing, the likely inherent incompatibility of polished platinum surfaces must be overcome, and that this to
Considering this to be the main reason why MEAC hydrogen sensors are not commercially available or used, the prior art of MEAC hydrogen sensing has substantially focused on modifying the anode surface. Given the prior art assumption that the activity or reactivity of the sensing anode must be increased, there is no obvious reason to consider the role of the membrane in more detail. This is because the above-mentioned U.S. patent no.
No. 3325378 describes the types of membrane materials and membrane thicknesses traditionally used for oxygen sensing, e.g.
It was believed that 25.4 μm Teflon (PTFE type or polytetrafluoroethylene), polyethylene, and polypropylene would be suitable for hydrogen sensing. Instead of employing arbitrary membranes and attempting to optimize anode activity or performance, the present invention begins with an optimal (i.e., polished) platinum anode surface and optimizes membrane performance. This is based on the following considerations. The role of the anode and membrane in the MEAC is that the magnitude of the current must on the one hand correspond to the net amount of hydrogen flowing through the membrane into the sensor (at steady state and expressed in suitable units), and On the other hand, it can be clarified by assuming that it must correspond to the rate of disappearance of hydrogen due to reactions (oxidation) at the anode. The prior art experience that an increase in anodic activity is required to obtain stable results is reproduced in the sense that at the cost of increasing anodic reactivity (i.e., the rate of hydrogen reaction at the anode), hydrogen diffusion through the membrane can be reduced. can be interpreted. As shown below, the validity of reversing the interpretation as described above can be proven both experimentally and mathematically, and the result of this reversal is that hydrogen with good responsiveness and stability If the measurement uses a polished platinum anode, the operation of the MEAC is controlled by the membrane (diffusion or permeation of hydrogen through the membrane is the limiting factor) but not by the reaction (hydrogen reaction at the anode). contrary to the teachings of the prior art). The term "determination of elemental hydrogen in a fluid medium" includes, for example, dissolved in liquids (including water and aqueous or non-aqueous media) or other gaseous fluids (e.g.
From the detection of small or trace amounts of gaseous hydrogen, e.g. in ppb, ppm or %, mixed with hydrogen halides, ammonia, water, steam and mixtures thereof, including hydrogen halides, ammonia, water, steam and air, to the determination of the purity of gaseous hydrogen in the range e.g. 50-100%. Within the range of measurement,
Includes various formats for measuring hydrogen content or concentration in a fluid medium. The term "amperometric cell" means a cell of the MEAC or Clark type, as already explained with reference to a number of patents. The expression "consisting of at least two electrodes" means that the cell has at least one hydrogen sensing electrode that is an anode for hydrogen sensing and at least one counter electrode that is a cathode for hydrogen sensing. The cell may also have a protective electrode of the type disclosed in DE 2851447, which is used in the present invention in substantially the same unit as the hydrogen sensing electrode to protect it. Although each electrode may consist of two or more segments, constructing the electrode from segments is generally not preferred for the anode. The counter electrode serves as a reference point when measuring the potential of the hydrogen sensing electrode. Preferably, the cathode surface area is greater than the anode surface area by a factor typically in the range of 2 to 10. The term "in contact with an aqueous electrolyte" generally means that the surface of each electrode is exposed to the electrolyte. The electrolyte-exposed surface area or sensing surface of the hydrogen sensing electrode is made of platinum metal, as described below, and must remain unaltered. The electrolyte exposed surface of the counter electrode is preferably substantially larger than the sensing surface of the hydrogen sensing electrode, for example by a factor of at least 3, preferably at least 5, although this need not be fully specified. Silver/silver chloride, silver/silver ions and various redox systems such as iodine/iodine or noble metal cathodes (eg gold or platinum)
Conventional cathode materials including ferrous/ferric as exchanged above are generally suitable. Generally, the counter electrode must meet the criteria that the product is non-oxidizing at the hydrogen sensing electrode and does not interfere with cell operation. Suitable aqueous electrolytes include those disclosed in the aforementioned patents and publications relating to hydrogen sensing, such as the chloride electrolyte compositions disclosed in U.S. Pat. No. 3,325,378, which are suitable; There is no need for buffering for this purpose and standard solutions of aqueous inorganic acids such as hydrochloric acid and sulfuric acid are included. Alternatively, an acidic electrolyte consisting essentially of the aqueous inorganic acid can be used as long as the acidic hydrogen ion concentration does not degrade the MEAC components. The term "reaction impedance" refers to the reaction impedance of a hydrogen sensing electrode and can be quantified as the reciprocal of the rate of hydrogen (oxidation) reaction at the hydrogen sensing electrode (H ₂ →2H ⁺ +2e, where e is (showing electrons at the anode). However, in practical applications of the invention, such quantification is not required. A high value of reaction impedance, of course, corresponds to a slow rate of hydrogen oxidation at the hydrogen sensing electrode. The term "membrane impedance" refers to the hydrogen diffusion impedance provided by a membrane and can be quantified as the inverse of the rate of hydrogen diffusion (permeation) through the membrane. Although such quantification is not necessary when actually using the present invention,
Specific data for the hydrogen permeability of many commercially available polymeric films is available or can be readily measured. To carry out the method of the present invention, the membrane impedance sufficiently exceeds the reaction impedance to provide membrane-controlled cell operation (hereinafter referred to as "M/C operation"), i.e., reaction-controlled cell operation (hereinafter referred to as "M/C operation"). It is important to prevent "R/C operation") and maintaining this requirement is essential to the present invention as a corollary to using a hydrogen sensing electrode with a smooth sensing surface of platinum metal. In view of this, for many purposes of the present invention, it is necessary to check the results, i.e. maintain M/C operation or R/C using a given MEAC with a given polished platinum electrode and a given electrolyte. It is sufficient to check for avoidance of manipulation. A first convenient method is to determine the dependence of the current of a given sensor on the partial pressure of hydrogen in the ambient or test gas (hereinafter referred to as "current/pressure dependence") for membranes made of different materials and/or different thicknesses. ) is to be measured and recorded. For reasons that become clear from the numerical model explained in the examples, the current/pressure dependence is linear only in the M/C operation according to the invention. moreover,
Diffusion theory [G.M. Hale et al., Journal of Electroanalytical Chemistry, Vol.
107 (1980), pp. 281-294] and the measurement of the response time of the MEAC to a step change in hydrogen, described in detail below, shows that the MEAC
shows a linear dependence of the reduced time parameter on time only when operated in the R/
In the case of the C operation, the dependence of the reduced time parameter on time is clearly non-linear. In contrast, the time dependence or drift (i.e., negative stability) of other similarly obtained hydrogen measurements
can be used to prove the M/C operation according to the present invention. For example, when repeated measurements are taken, the drift toward lower values is generally less than 1% per day in M/C operations and greater than 10% per day in R/C operations. The provision to indirectly control M/C operations relative to R/C operations is advantageous. This is because the actual hydrogen permeation is determined by the specific nominal hydrogen permeability of a given organic polymer film (often in molar units such as membrane surface area, pressure difference and μmol of hydrogen per hour).
expressed as μmol·m ^-2 ·(Kg/m ² ) ^-1 ·sec ^-1 ) and specific nominal thickness (e.g. μmol·m -2·(Kg/m 2 ) -1·sec -1)
m), but also on incidental factors such as intentional or accidental stretching of the membrane. Although a number of non-limiting examples of organic polymers commercially available as films of various dimensions and suitable for the present invention are shown in the table below, it is understood that organic polymer films of essentially specific thicknesses are suitable. Ru. That is, it is the actual "permeability value" of the membrane that essentially determines the rate of hydrogen diffusion. "Permeability" is a unit often given without reference to the actual membrane thickness, and the "permeability coefficient" given below relates to units of dimension or thickness (μm). Generally suitable membranes are less than 9.8 x ¹⁰² , preferably
9.8×10 (μmol・μm・m ^-2・(Kg/m ² ) ^-1・s ^-1 )
It has a hydrogen permeability coefficient of less than 9.8×
Membranes with permeability coefficients of less than 10 can be used in typical thicknesses of 25.4 μm or less, such as up to about 10 μm. Membranes with permeability coefficients in the range 9.8×10 to 9.8×10 ² are suitable for use with higher membrane thicknesses (i.e. 50 to 150 μm).
The membrane pressure is generally less than 300 μm, preferably less than 100 μm, particularly preferably less than 50 μm, for many purposes of the invention where it is important to respond relatively quickly to step changes in hydrogen partial pressure in the surrounding fluid. Let's understand what should be done. On the other hand, very thick membranes of about 1 mm can be used with membrane materials having high hydrogen permeability (eg silicone rubber). However, halogen-containing polymers that provide low permeability coefficients are suitable for many purposes. Specific products include, but are not limited to:

[Table] Other examples are polyvinylidene fluoride (symbol: PVDF),
Poly(monochlorotrifluoroethylene) (symbol: PCTFE), poly(tetrafluoroethylene)
(Symbol: PTFE) and other polymers. The term "platinum group metals" refers to metals of the "platinum group" such as Pt, Pd, Rh, Ir, Ru and OS,
It also includes alloys or mixtures of these metals or these metals with noble metals such as gold. Substantially pure platinum is suitable and preferred for purposes of the present invention. The electrolyte used in the method of the invention is aqueous.
Aqueous solutions of salts of inorganic or organic acids and inorganic or organic bases with concentrations in the normality range, for example from 0.5 to 2N, are suitable. Preferably, the electrolyte selected should contain as few anions as possible, taking into account factors such as MEAC parameters such as compatibility, corrosion resistance, etc. This is because such anions are adsorbed in small amounts on the surface of the hydrogen sensing electrode, which is therefore suitable for stable operation as explained below. The order of polarity (from high to low) can be shown as follows: I ^- ＞Br ^- ＞C1 ^- ＞SO ₄ ^-- , C1O ₄ ^- ,
F- ^. For example, water-soluble fluorides or perchloric acid are often preferred over iodides, bromides and chlorides, and salts that provide low resistance are preferred, with sodium fluoride being a suitable example. Another factor that can influence the degree of anion adsorption is the voltage applied between the hydrogen sensing electrode (anode) and the counter electrode (cathode). Generally, in the method of the present invention, it is preferred to keep the adsorption of anions to the hydrogen sensing electrode as low as possible. This is because such adsorption competes with the adsorption of hydrogen at the hydrogen sensing electrode (electroactive species should be measured by electrooxidation). Each hydrogen molecule must be dissociated into two adsorbed hydrogen atoms in order to be oxidized, and increasing adsorption of anions from the electrolyte reduces the number of reactive hydrogens. Therefore, if the voltage-dependent increase in anion adsorption becomes significant and the observed current decreases with increasing voltage (instead of increasing exponentially with increasing voltage), then A dependent increase in anion adsorption can be detected. As a result, the voltage used for polarization is preferably chosen as low as possible, and the range of operable currents is specified in such a range that the current is linearly related to the ambient hydrogen partial pressure. The term "substantially smooth" as used herein to describe the surface characteristics of a sensing surface is defined as a "smooth" surface, typically defined by a high reflectance of 50% or higher.
Meaning "shining" or "polishing". Unlike conventional anodes for hydrogen sensing that have "rough" or "black" surfaces and where the actual surface area is hundreds of times the apparent surface area, the typical smooth hydrogen sensing electrode surface of the present invention is , obtained by polishing a metallic platinum surface until a shiny, materially mirror-like surface appearance is obtained. Typically the actual surface area of a smooth surface is less than 3 times the apparent surface area, typically 1
~2 times or less. A suitable method for determining this ratio is known per se, by measuring the capacitance of the surface to be tested at the voltage at which hydrogen is evolved by reduction of hydrogen ions, and comparing the obtained value to that of a perfectly smooth mercury electrode. [Article by E.C. Potter, Electrochemistry, London, p.
1956]. Other suitable methods are charge measurements or adsorption methods, which indicate the actual area rather than the apparent area. The term "applied voltage" means the potential difference applied between the hydrogen sensing electrode and the counter electrode. It appears that the desired M/C operation to obtain a given membrane impedance can be easily achieved in this way, since the reaction rate can be increased and the reaction impedance can be decreased by increasing the applied voltage. However, the threshold for the oxidation of water (which is part of the electrolyte) is quickly reached ((1.23 V). This threshold represents the limit for reducing the reaction impedance. Furthermore, as mentioned above, it should be reduced to zero as much as possible. Polarization at close voltages tends to minimize undesirable anion adsorption and is preferred to maintain an acceptable current range. Typical currents when operating the method of the invention are:
Depends on the nature of the electrolyte. For example, when operating with 1N sulfuric acid (aqueous), the hydrogen sensing electrode area is 1 mm ²
Currents of up to 3 μA or more per hydrogen partial pressure of 1.02 Kg/cm ² are observed. For chloride electrolytes, lower current levels are observed, increasing the membrane impedance to the point that the current per 1.02 Kg/ ^cm2 of partial pressure ( ^H2 ) is maintained below 1 μA per ^mm2 of hydrogen sensing electrode area. It is desirable to do so. As mentioned above, acceptable measurement stability during hydrogen measurements according to the invention means a drift of less than 1% per day. Another advantage of the M/C operation according to the present invention is that the M/C operation according to the present invention is typically about 1000 ml/min, which is well below that required for conventional hydrogen sensing.
This may lie in the fact that flow rates (ambient fluid) of, for example, 200-300 ml/min can be achieved. MEAC devices of various types or of various origins can be used that are suitable for use with the present invention.
A preferred type is that described in the aforementioned U.S. Pat. No. 3,096,047 (pressure seal), in which the central electrode is coated with polished platinum,
4325797 (membrane holding device) and with or without the variant disclosed in DE 2851447 mentioned above (protected electrode). Commercially available MEAC equipment is located in Geneva, Switzerland.
It is commercially available as Model No. 2230 by Orbisphere Laboratories, Inc., in which the central electrode is provided with a 300 μm layer of platinum by rolling a platinum sheet onto a rigid substrate. Other materials commonly required for current measurements are also commercially available, including voltage sources, temperature compensation means, amplifiers, recording devices, electrolytes, etc., and suitable membranes can be cut from commercially available polymeric films of the type described above. I can do it. Hereinafter, the invention will be further explained by way of example with reference to the accompanying drawings. FIG. 1 is a cross-sectional view showing the structure of a sensor 10 of a known type for oxygen detection. Sensor 10 is generally cylindrical and includes the following major components in a generally coaxial arrangement: The outer case 11 holds the membrane 12 at its upper end by a retaining ring 13 and surrounds a counter electrode 14 as a cathode counter electrode, which is used for hydrogen detection (i.e. for electrolytic oxidation of elemental hydrogen as described above). ) becomes the cathode. An electrically insulating layer 15 separates the counter electrode 14 from an adjacent electrode 16, which in this description is a suitable protective electrode maintained at approximately the same potential as the sensing electrode 18, which is the central anodic hydrogen sensing electrode and is a sensing electrode. The electrode 18 serves as an anode during hydrogen detection. An insulating layer 17 is placed between the protective electrode 16 and the sensing electrode 18.
An electrolyte reservoir 19 is provided to contain an aqueous electrolyte of the type described above for hydrogen sensing. As is known in the art, the electrolyte forms a continuous and preferably very thin layer between the membrane 12 and the electrolyte-exposed sensing surface 180 of the sensing electrode 18. For purposes of the present invention, sensing surface 180 is metallic platinum,
For example, it is made of pure platinum and is substantially smooth (eg, polished) to achieve a reflectance of at least 50%, which corresponds to a mirror-like surface quality. Since the entire sensing electrode 18 may be made of metallic platinum, a surface layer having a thickness on the order of micrometers (for example 5 to 50 micrometers) or mm (for example 0.1 to 1 mm) is generally applied to the substrate by, for example, rolling or soldering techniques. It is sufficient to apply it to In both oxygen and hydrogen sensing, unlike the prior art, the membrane 12 is not selected for high permeability to the electroactive species of interest, but rather has a membrane 12 of limited, preferably low hydrogen permeability. Used as described above to achieve membrane limited (M/C) cell reactions, which occur when the membrane impedance significantly exceeds the reaction impedance. FIG. 2 is a graph showing various current/pressure curves obtained with a sensor (Orbisphere Laboratories, Model 2230) as described above. The platinum surface of the sensing electrode 18 was polished to specular brightness using a polishing paste with a particle size of less than 0.5 μm. The membranes used had a thickness of 12.7 μm (12,7 μm, curve A; 25.4 μm, curve B) and 127 μm (127 μm, curve A).
It was constructed with commercially available FEP (listed in the table) of curve C).
On the vertical axis, the current i (typically μA) is plotted, and on the horizontal axis, the partial pressure P=(fugacity) of hydrogen in the surrounding fluid as fluid medium is plotted in units of 0.102 Kg/cm ² . The electrolyte used was a 1N sulfuric acid aqueous solution. The ambient fluid outside the membrane 12 is between 0 and 1.43
A gaseous mixture of nitrogen and hydrogen was prepared with a hydrogen fugacity of Kg/cm ² . Curve A shows that 12.7 μm FEP membranes are not suitable for the method of the invention. Because after a relatively small increase in hydrogen fugacity, about 130μA
This is because the "saturation" current, or horizontal portion, is reached at . After reaching this plateau, further increases in hydrogen fugacity did not result in significant current changes. This indicates R/C operation based on the dominance of reactive impedance. Curve B, measured similarly to curve A except using other film thicknesses, shows that FEP films with twice the thickness (up to 25.4 μm) improve the current/fugacity proportionality. However, this indicates that the membrane impedance is not increased sufficiently to provide linearity of the current/fugacity relationship indicative of M/C operation. Curve C in Figure 2 shows that a specific membrane material FEP with a thickness 10 times that of curve A (127 μm) is sufficient to achieve a strict current/fugacity linear relationship, i.e., the reaction impedance is This shows that the membrane impedance is sufficiently dominant to achieve M/C operation. At least theoretically, the membrane material can handle a total of three types of current/current, corresponding to curves A, B, and C in FIG.
It is clear that a fugacy (hydrogen) relationship is shown. However, less suitable materials exhibit linearity only at membrane thicknesses that are too large for practical purposes (curve C), whereas preferred membrane materials with very low hydrogen permeability coefficients can be used for hydrogen sensing according to the present invention. In this case, linearity is always exhibited at film thicknesses of 25 μm or 10 μm. The above transmission coefficients are given for comparison purposes and quick orientation purposes. The methods used for these measurements are described below, but other methods can also be used. The film thickness is expressed in μm because it is often used for industrial convenience. According to an embodiment of the invention, the sensor 10 constitutes an amperometric cell with a reactive impedance;
The sensor 10 includes a sensing electrode 18 and a counter electrode 14.
and a membrane 12 that is permeable to elemental hydrogen but substantially impermeable to electrolyte and provides a membrane impedance. The sensing electrode 18 has a substantially smooth sensing surface 180 made of a platinum group metal, and the sensing electrode 18 has a substantially smooth sensing surface 180 made of a platinum group metal.
80 has an apparent surface area to actual surface area ratio ranging from 1:1 to 1:3. The membrane impedance is selected to sufficiently exceed the reactive impedance to effect membrane control operation of sensor 10. The quantitative electroanalytical measurement method for elemental hydrogen according to this embodiment includes at least two electrodes that are in contact with an aqueous electrolyte and provide a reactive impedance of the sensor 10, and that are permeable to elemental hydrogen but not electrolytic. The electrolyte is separated from the surrounding fluid by a membrane 12 that is substantially impermeable to the liquid, and the sensor 10, given the membrane impedance of the sensor 10, quantitatively and electroanalytically detects elemental hydrogen in the surrounding fluid. , the at least two electrodes include a sensing electrode 18 and a counter electrode 14, the sensing electrode 18 has a substantially smooth sensing surface 180 made of a platinum group metal, and the sensing electrode 18 has a substantially smooth sensing surface 180 made of a platinum group metal. is 1:1~
With an apparent surface area:real surface area ratio in the range of 1:3, the membrane impedance is selected to sufficiently exceed the reactive impedance to effect membrane control operation of the sensor 10. The present invention will be further explained below with reference to Examples. Example 1 The hydrogen permeability coefficient of the membrane material shown in the table above is
Measurements were made using the model 2230 (Orbisphere Laboratories, Inc.) previously described in connection with FIGS. 1 and 2. The electrolyte was a 1N sulfuric acid aqueous solution, and a constant DC voltage of 0.5 volt was applied between the hydrogen sensing electrode made of polished platinum and the Ag/AgC1 counter electrode. Next, to determine the hydrogen permeability value (HPV) of the membrane material, a test membrane material of known thickness is installed on the sensor, and this sensor is tested for the hydrogen permeability value (HPV) of the membrane material with a known thickness (1.02Kg/
cm ² ) in a test atmosphere, measuring the passage of current through the sensor, and calculating the HPV from the following relationship: HPV = Current (clones/second)/Membrane thickness (m)
/electrode area (m ² )・2 furadays (clones/mole)・
H ₂ Pressure (Kg/cm ² ) The reason why the factor 2 is included in the denominator is that a charge of 2 Faradays is added to 1 mole of molecular hydrogen during the electrode reaction. 25.4μm ETFE film (thickness 25.4×10 ^-6 m) is F=
1.02Kg/cm ² using 9.64867×10 ⁴ clones/mol
A current of 37.5×10 ⁻⁶ clones/second (37.5 μA) was generated for the electrode area (3.142×10 ⁻⁵ m ² ) at a hydrogen pressure of . In other words, HPV is 1.54×10 ^-10 (mol・m/
^m2・Kg/ ^cm2・sec). A film of other materials with a thickness of 25.4μm and a film of 1.02μm
The following current (μA) was measured using a hydrogen pressure of Kg/cm ² . PVF2.5, PFA140, FEP100 and E-CTFE3.61. The hydrogen permeability coefficients calculated from these values are summarized in the table above. Example 2 Polished platinum disk-shaped hydrogen sensing electrode (diameter
6.32 mm) and a silver chloride cathode (counter electrode) was filled with an electrolyte containing 1 mole of KC1 and 1 mole of HC1 per distilled water, and was made of 12.7 μm PVF. The membrane was attached. 0.7V
was applied between the hydrogen sensing electrode and the counter electrode of the sensor, and the current in an atmosphere of H ₂ partial pressure of 1.02 Kg/cm ² was thus found to be 5 μA. The relationship between measured current and hydrogen fugacity is from 1.5ppb to 1.02Kg/ ^cm2 to 5.10Kg/ ^cm2 )
It was found to be linear over a range of up to 7.5 ppm (corresponding to the range of dissolved hydrogen concentrations in water at 25 °C). The response time at 25°C was measured to be 5.2 seconds for 90% of the total signal change. The downward drift rate is less than 1% per day.
Recordings were made over a period of one week when the sensor was exposed to a constant atmosphere of hydrogen saturated with water at a total pressure of 1.02 Kg/cm ² . Flow rates slightly above 200 ml/min have been found to be suitable for valid measurements. Example 3 This example has the purpose of showing that the requirements when a hydrogen measurement sensor is used for a specific application affect the range of hydrogen partial pressures to be measured and the allowable reaction time. Some simple experiments confirmed whether the selected membranes met these requirements. Certain membranes are not suitable because they respond too slowly, or because they are impermeable and the detection threshold is higher than the lower limit of the pressure range to be measured. This is demonstrated below using an unusually thick film. The sensor of Example 2 was filled with 1N sulfuric acid as an electrolyte and equipped with membranes of FEP, ETFE, and E-CTFE with a thickness of 127 μm. In each case, when exposed to a known partial pressure of hydrogen, the sensor Its sensitivity was determined by measuring the current generated. These results are recorded in the first place. In addition, the response of the sensor to stepwise changes in hydrogen partial pressure was measured and analyzed according to diffusion theory ( (See ^Example 4 ₎ .The characteristic ^time _τ = When exposed to pure nitrogen,
The measured current was 1.5 nA regardless of the film properties. Therefore, a hydrogen partial pressure that produces a sensor current of less than 1.5 nA is below the detectability threshold. For example, for a sensor equipped with a 127 μm thick FEP membrane, this threshold value is as follows. 1.5 (nA)/19700 (nA/Kg/cm ² )=7.65×10 ⁻⁵ Kg/cm ² These threshold values are also shown in the table.

[Table] Example 4 In the sensor of the above example, (A) 127 μm thick ETFE
(B) 12.7 μm thick ETFE film, (C) 127 μm thick
(D) an ECTFE membrane with a thickness of 127 μm and (D) an FEP membrane with a thickness of 127 μm. The responses were measured at 25°C and the results analyzed in terms of time/response behavior. According to diffusion theory, after a rapid stepwise change in hydrogen fugacity, the sensor current i observed at various times t in M/C operation is expressed as: i-i ₀ /i _∞ −i ₀ =1+2 _∞ 〓 ⁿ⁼¹ (-1) ⁿ exp[-n ² π ² t/τ] [where i ₀ is the initial current, i _∞ is the final steady state current, and τ=X ² _n /D _n is, It is the characteristic time for the passage of dissolved gas molecules through the membrane. ] [Hale et al., Journal Electroanalytical
Chemistry, Vol. 107, (1980), p. 281]. This therefore provides a simple method of analysis of experimental current/time data that can distinguish between M/C and R/C operations, and a rapid test can determine the time at which the sensor reaches various parts of the total signal change. is to compare: i−i ₀ /i∞−i ₀・100 = n% of total signal change Some of these times are: 25% 0.094τ 50% 0.138τ 63.2% 0.170τ 75 % 0.210τ 90 % 0.305τ 95 % 0.375τ 99 % 0.530τ Therefore, the ratio of time for 25% change to time for 75% change is: 0.210τ / 0.094τ = 2.23 A similar ratio is If found to fit the entire instantaneous response, the current should be diffusion controlled, and the linearity of the t/τ function when operating the MEAC for hydrogen measurements is required for the method of the invention. Sensor M/
C operation is shown. The following table shows the range from 0.102Kg/cm ² to 0.306Kg/cm ² (25
Normalized current i−i ₀ −/i∞−i ₀ (“A−1”) for a 127 μm thick ETFE film measured in response to a hydrogen fugacity step at -2'') and the corresponding converted time or t/τ value (``A-3'').

[Table] Since A-3 shows almost linear behavior,
MEAC's 127 μm ETFE membrane provides M/C operation for hydrogen measurements according to the present invention. This measurement was repeated and the analysis was carried out under the same conditions as in (A) except that a 12.7 μm ETFE membrane (B) was used, and the results shown in the table below were obtained.

[Table] B-3 shows non-linearity, which is 127μm thick.
The same membrane material ETFE that was suitable in the table for the method of the invention when used in
M/C according to the invention when used at 12.7μm
It has been shown that this does not provide a sufficiently high membrane impedance as required for operation. Further, similarly ^{, t} / The linearity of the τ versus t function was confirmed. Example 5 For purposes of numerical analysis according to the teachings of the present invention, an equation is established for a sensor in which the membrane impedance and the electrode response impedance are approximately the same magnitude. illustrating on the one hand the fact that, at steady state and expressed in suitable units, the magnitude of the current must be equal to the net rate of hydrogen flowing through the membrane into the sensor; On the other hand, two equations are used to illustrate the fact that the magnitude of the current must be equal to the rate of electrochemical disappearance of hydrogen from the electrolyte space adjacent to the electrode. Therefore, (1) i=2FAS _n D _n (P ₀ −P _i )/X _n and (2) i=2FAK(P _i ⁾ nwhere, i Current F Faraday A Electrode area S _n in the film Hydrogen solubility D _n Diffusion coefficient of hydrogen in the membrane P ₀ Partial pressure of hydrogen acting on the outside of the membrane P _i Partial pressure of hydrogen in the electrolyte between the membrane and the hydrogen sensing electrode K Partial pressure of hydrogen in the hydrogen sensing electrode Kinetic rate constant for electrolytic oxidation n Order of oxidation reaction on dissolved hydrogen X _n Film thickness The absolute maximum value of hydrogen partial pressure inside the sensor is equal to the hydrogen partial pressure outside, i.
The upper limit for P _i (max) = P ₀ current must be i ₁ = 2FAKP ⁿ ₀ . P _i can then be eliminated from the equation. This is because P _i =P ₀ (i/i1) ^1/n . Thus, the above equation (1) becomes as follows. i=2FAS _n D _n P ₀ /X _n [1-(i/i1) ^1/3 ] This means that either the membrane impedance or the reaction impedance, or both of these impedances control the magnitude of the sensor current. is the desired formula that describes the characteristics of MEAC. For comparison, used as an oxygen detector
Create an equation for the magnitude of the current in the MEAC. Since the electrolytic reduction of oxygen is very rapid, ie K and i ₁ are ∞, i/i ₁ can be ignored. (3) i=2FAS _n D _n P ₀ /X _n = i _dThis is the expression for the current in the MEAC limited by the diffusion of reactants through the membrane. Symbolized by i _d to emphasize the fact that diffusion has a rate-limiting influence. Further inspection of the first equation shows that the diffusion control limit is reached if equation (4) i ₁ ≫i _d or (5) S _n D _n /X _n ≪. When the electrode reaction rate is extremely slow (6) i ₁ ≪i _d , that is, (7) K≪S _n D _n /X _n , the magnitude of the current is determined by the dynamics of the electrode reaction and the equation (8) i→i ₁ =2FAKP ⁿ ₀ is determined. If n~0 (zero order with respect to the dissolved substance), the current is not a measure of the hydrogen concentration in this extreme case. The full formula applies to the full range of relative magnitudes of i ₁ and i _d . As a result, the kinetic current limit i ₁ becomes the membrane diffusion limit i _d
Only when the hydrogen partial pressure is sufficiently exceeded will the sensor generate an output proportional to the hydrogen partial pressure. In the art, this result is achieved by increasing the reaction rate to such an extent that the reaction impedance is sufficiently reduced. According to the invention, the membrane material and/or membrane thickness are selected such that the inequality S _n D _n /X _n <<K is satisfied even when, for example, polished platinum is used for the hydrogen sensing electrode surface.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of a hydrogen sensor embodying the invention, and FIG. 2 is a graph of a number of curves showing different types of dependence of current on hydrogen partial pressure. 10...sensor, 11...outer case, 12...
...Membrane, 13...Retaining ring, 14...Counter electrode,
15... Electrical insulating layer, 16... Protective electrode, 17...
...Insulating layer, 18... Sensing electrode, 19... Electrolyte storage tank, 180... Sensing surface.

Claims (1)

Claims: 1. At least two electrodes in contact with an aqueous electrolyte to provide the reactive impedance of the amperometric cell, permeable to elemental hydrogen but substantially impermeable to the electrolyte. A method for quantitatively and electroanalytically measuring elemental hydrogen in a fluid medium by separating said electrolyte from said fluid medium by a membrane, and using said amperometric cell having a membrane impedance of said cell. wherein the at least two electrodes include an anode hydrogen sensing electrode and a cathode counter electrode, the hydrogen sensing electrode has a substantially smooth sensing surface made of a platinum group metal, and the smooth sensing surface has a 1:1 ratio. an element having an apparent surface area:real surface area ratio in the range of ~1:3 and selected such that the membrane impedance sufficiently exceeds the reaction impedance to effect membrane control operation of the amperometric cell. Quantitative electroanalytical measurement method for hydrogen in the form of hydrogen. 2. The method of claim 1, wherein the membrane-controlled operation of the cell is evidenced by an approximately linear current/pressure dependence. 3. The method of claim 1, wherein membrane controlled operation of the cell is evidenced by a downward drift of less than 1% per day. 4 The membrane has a density of 9.8×10 ² (μmol・μm・m ⁻² (Kg/
m ² ) ^-1・s ⁻¹ ), preferably less than 9.8×10 (μmol・μ
2. A method according to claim 1, having a membrane impedance corresponding to a hydrogen permeability coefficient of less than m·m ⁻² (Kg/m ² ) ⁻¹ ·s ⁻¹ ). 5 The membrane has a thickness of less than 300 μm, preferably 10 to
5. A method according to any one of claims 1 to 4, consisting essentially of a halogen-containing polyene film having a thickness in the range of 125 μm. 6 The membrane is made of polyvinyl fluoride, a copolymer of ethylene and tetrafluoroethylene, polyvinylidene chloride, a fluorinated ethylene/propylene copolymer,
Polyfluoroalkoxy polymers, copolymers of ethylene and monochlorotrifluoroethylene, polyvinylidene fluoride, poly(monochlorotrifluoroethylene) and poly(tetrafluoroethylene)
6. The method according to any one of claims 1 to 5, wherein the film is a polymer selected from the group consisting of: 7. The method according to any one of claims 1 to 6, wherein the film has a thickness of 10 to 50 μm. 8 an amperometric cell comprising an anodic hydrogen sensing electrode and a cathodic counterelectrode in contact with an aqueous electrolyte and having a reactive impedance and permeable to elemental hydrogen but substantially impermeable to the electrolyte; and a membrane providing a membrane impedance, the sensor for quantitative electroanalytical measurement of elemental hydrogen in a fluid medium, wherein the anodic hydrogen sensing electrode has a substantially smooth sensing surface made of a platinum group metal. and the smooth sensing surface has an apparent surface area:actual surface area ratio in the range of 1:1 to 1:3;
The membrane has a thickness in the range of 10 to 300 μm and a thickness of 9.8×10 ²
A sensor for quantitative electroanalytical measurement of elemental hydrogen having a hydrogen permeability coefficient of less than (μmol·μm·m ⁻² (Kg′/m ² ) ⁻¹ ·s ⁻¹ ). 9 The membrane has a thickness in the range of 10 to 125 μm, preferably
A sensor according to claim 8 having a thickness in the range 10-50 μm. 10 The membrane is made of polyvinyl fluoride, a copolymer of ethylene and tetrafluoroethylene, polyvinylidene chloride, a fluorinated ethylene/propylene copolymer, a polyfluoroalkoxy polymer, a copolymer of ethylene and monochlorotrifluoroethylene. Union,
10. The sensor according to claim 8 or 9, which is a film of one polymer selected from the group consisting of polyvinylidene fluoride, poly(monochlorotrifluoroethylene), and poly(tetrafluoroethylene).